How Masks Works in Photoshop

What is a mask?



Essentially a mask is a grayscale bitmap image.The pure white areas in the image represent the portions of your original image that will be 100% protected. The pure black portions of the image represent the areas of your original image that are completely masked out, or erased. The levels of gray in-between allow your image to be partially protected. If you have trouble keeping track of which color does what, just think of masking tape to help you remember... masking tape is usually white or light colored, so the white areas of your mask are the most protected. Many image editing software allows you to save your masks as grayscale images so they can be used over and over again. On the following pages, I'll show you how you you can use any grayscale image as a mask in several popular image editors.

this is a mask

this is an image

this is an image with a mask

In the examples above you can see a mask and how it interacts with the image. Compare the masked image to the mask. Notice that the areas of white in the mask are 100% opaque in the masked image. The areas of black are 100% transparent. The gray areas softly fade away in partial transparency.

A mask does not have to be a separate image. Most software allows you to paint a mask directly onto your image. It works the same way... painting with black while in mask mode erases the underlying layer of your image, and painting with white brings it back. Shades of gray allow you to paint in partial transparency.

 When you're painting in mask mode, the mask is usually represented on the screen by a ruby overlay. The ruby overlay lets you see the mask represented by a reddish tint while still allowing you to still the image you're working on. Some software allows you change the overlay tint color. To the right you can see the same image used in the examples above with a ruby overlay. Transparency is indicated by a checkerboard pattern in image editing software; that's why you see the checkerboard pattern in this image.

Source : http://www.about.com